Presently, such optical rangefinders are used when distances of a few 100 meters are to be measured accurately and rapidly. Because there are no electronic detectors capable of directly registering the frequency of visible or infrared measuring light, these distance-measuring devices operate with amplitude-modulated light rays.
Examples of such measuring devices are the electronic tachymeters disclosed in "Zeiss-Informationen", Vol. 20, No. 80, page 24 (1972). These have a relatively simple optical configuration: a light-emitting diode (LED), operated via a frequency-stabilized electronic oscillator and an amplifier, generates an amplitude-modulated light beam. It is directed at the target, reflected by the latter, and received again by the device and detected. The detector signal and a reference signal from the amplifier are electronically mixed with the signal of another frequency-stabilized electronic oscillator. Then the phase difference between the detector signal and the reference signal is determined.
The measuring accuracy of such prior art tachymeters is approximately 1 mm over a measuring distance of a few 100 meters. This corresponds to a relative measuring accuracy of 5.times.10.sup.-6. Further, only specularly-reflective bodies (as distinguished from diffusely-reflective bodies) are suitable as targets.
PCT Application No. WO 88/08519 (Dandliker) discloses a measuring device which uses a heterodyne interferometric arrangement and is capable of even greater measuring accuracy, but over shorter measuring distances. This device uses two frequency-stabilized laser beams of different frequencies. Using two acousto-optical modulators driven, respectively, by an electronic oscillator, a second laser beam is generated from each of the two laser beams. Each of these second beams is then superimposed upon its related initial beam to provide two amplitude-modulated laser beams which, in turn, are both superimposed on each other to form the measuring beam. The inventors of this prior art measuring device have confirmed that its accuracy is a function of the target, the accuracy being lower with diffusely-reflecting targets than with specularly-reflected targets. [See R. Dandliker et al., Optics Letters, Vol. 13, No. 5, page 339 (1988)].
It is also known to utilize the high spectral sharpness of multi-mode lasers in interferometric linear measurement, and such interferometric devices provide measurements with a relatively high accuracy, e.g., greater than 10.sup.-6. Different, and sometimes very simple, methods of stabilizing spectral line sharpness in such lasers are well known, some assuring a spectral accuracy of .DELTA.f/f&lt;10.sup.-7. A few of these known methods are described in "Dokumentation Laserinterferometrie der Langenmesstechnik" (documentation of laser interferometry in linear measuring technology), published in VDI-Verlag Dusseldorf, 1985, page 8 and the following. Also, German Patent 20 43 734 discloses a particularly simple stabilizing method in which both modes of a two-mode gas laser are divided by polarization optics and their intensities are measured. A closed-loop control circuit controls the laser-initiating discharge current in such a manner that the intensity ratio of both modes remains constant. Nonetheless, all such known interferometric devices using multi-mode lasers require a highly reflective target, usually in the form of triple reflectors; and when diffusely-reflecting targets are used, the intensity of the reflected light is very low, and the phase of the measuring light beam is decorrelated following the diffuse reflection.
Quite generally, the relative measuring accuracy of such optical devices depends upon (a) the accuracy of the measurement of the relative phase difference between two measuring signals, and upon (b) the spectral stability of the amplitude-modulated light. If a distance (s) is to be determined with an accuracy of (.DELTA.s), the unsharpness of the spectral line (.DELTA.f/f) of the modulation frequency (f) must not be greater than the desired lateral accuracy (.DELTA.s/s), i.e.,: EQU .DELTA.f/f .ltoreq..DELTA.s/s
Therefore, if a measuring accuracy (.DELTA.s) of 0.1 mm is desired over a distance (s) of 300 meters, the electronic measurement of the phase shift must be based upon a spectral line accuracy (.DELTA.f/f) of at least 3.3.times.10.sup.-7. In order to obtain frequencies with such spectral accuracy, very complex and expensive oscillators are required.
The invention herein is a measuring device of the above-mentioned type which provides identical measuring accuracy with diffusely-reflecting, as well as specularly-reflecting, targets. In addition, since the inventive device omits the use of frequency-stabilized electronic oscillators, it is less expensive, and yet it provides measurements with a relative error of less than 10.sup.-6 with measuring times of less than 100 ms.